This invention relates to improvements in cutting tools and, in particular, to a cutting tool with an opening that facilitates accurate positioning of the axis of rotation of a cutting tool drilling a hole in a work piece.
Conventional tools, such as drill bits, for cutting holes in a work piece have pointed tips that position the axes of rotation of the drill bits at the locations of the centers of the holes to be drilled. However, a drill bit with a pointed tip and no means to ensure lateral stability tends to wander as the drill bit finds the center of the hole and thereby exerts a lateral force against its side wall. The result is a drilled hole of nonuniform diameter along its length, which prohibits a drill operator from drilling a pair of side-by-side holes with a thin partition between them. Drill bits of the type described in U.S. Pat. No. 5,832,720 do not suffer from lateral instability and are, therefore, capable of drilling dimensionally accurate, smooth-sided holes in work pieces.
FIGS. 1-5 show several views of a prior art cutting tool described in U.S. Pat. No. 5,832,720, in which a circular hole drill bit 10 comprises a cylindrical body 12 having a driver engaging end 14 opposite a work engaging end 16. A substantially cylindrical side wall 18 extends between driver engaging end 14 and work engaging end 16. Body 12 is adapted to be rotated about an axis of rotation 22 relative to a work piece in a predetermined direction indicated by a counterclockwise pointing arrow 20 when working. Work engaging end 16 may comprise a work end face 21 that is substantially flat (FIG. 1) or stepped (FIG. 1A) at work engaging end 16. Body 12 is formed with an elongate flute 24 in side wall 18, extending from work end face 21 towards driver engaging end 14 in a direction generally parallel to axis of rotation 22. At work end face 21, flute 24 has a minimum depth that is equal to the radius R of shaft 12. Flute 24 has a trailing face 30 with respect to direction 20 of rotation of bit 10 when rotated in its work direction. (FIG. 1A shows a flute 24 having a sharp corner at the step formation of face 21.)
Formed on work engaging end 16 of drill bit 10 is a cutting tooth 32 that is configured to have a cutting edge 34 and a guide projection 35 that extends laterally of side wall 18. Cutting edge 34 extends from axis 22 and along the portion of guide projection 35 nearer to work engaging end 16. Cutting edge 34 is also positioned above end face 21 as shown in FIGS. 1 and 4, so that cutting edge 34 may engage the work piece when drill bit 10 is rotated. Cutting edge 34 may extend perpendicularly to axis of rotation 22, in which instance cutting edge 34 terminates at axis 22, or cutting edge 34 may be inclined upwardly as is shown in FIG. 1 from axis 22 toward side wall 18, in which instance cutting edge 34 can extend beyond axis 22. In this latter instance, relief is provided on end face 21 at axis 22 so that the portion of cutting edge 34 extending beyond axis 22 does not counteract the hole cutting operation, as shown in FIGS. 1 and 2.
Guide projection 35 is formed with a guide face 50 that is concentric with axis 22 and intersects cutting edge 34 to form a sharp corner. Guide face 50 is spaced from axis 22 by an amount greater than the spacing of any other part of body 12, so that in rotation of drill bit 10 no portion of body 12 will engage the side wall of the hole formed in the work piece by drill bit 10. As a practical matter, the spacing should be slightly greater than the eccentricity, colloquially known as xe2x80x9cslop,xe2x80x9d in the rotation of the chuck holding the drill bit, which eccentricity results from slack in the mounting of the chuck in the driving mechanism. The guide projection distance from side wall 18 is also determined by the properties of the drill bit materialxe2x80x94the harder the drill bit material, the greater the possible guide projection distance without flexure of drill bit 10. Normally the guide projection distance from side wall 18 in a drill bit of 1 to 1xc2xc inches (2.5-3.2 centimeters) in diameter will be between about 0.001-0.250 inch (0.025-6.4 millimeters).
In the direction parallel to axis 22, guide face 50 preferably has a minimum length, L, equal to the depth of the cut made by cutting edge 34 in about one and one-half revolutions of the drill bit. This may be from 0.001-0.500 inch (0.025-12.7 millimeters) depending on the material being cut. In the circumferential direction, guide face 50 preferably also has a length of no less than the depth of cut. Guide face 50 preferably is maintained at a minimum size so as to minimize the area of contact with the work piece, thereby minimizing the amount of heat generated to not appreciably increase the temperature of the finished surface and to keep drill bit 10 cool. Cutting tooth 32 rearwardly of guide face 50 is tapered inwardly toward side wall 18 leaving enough material to support guide projection 35. This inward taper provides for guide face 50 relief that prevents scoring of the finished surface of the work piece by side wall 18 as drill bit 10 advances into the work piece to remove material from it.
When drill bit 10 is to be used to cut through layers of a composite work piece of different degrees of solidity or hardness (e.g., fibrous material and solid material layers), the length L is preferably chosen to be longer than the depth of cut achievable by at least one revolution of drill bit 10 in each of two adjacent layers so that guide projection 35 concurrently contacts the two adjacent layers while cutting through the transition line between them. This is to ensure layer-to-layer overlap of guide projection 35 and thereby prevent deflection of the cutting path of drill bit 10 at the transition between adjacent work piece layers.
FIG. 2A is an end view of an alternative circular hole drill bit 10a, which differs from drill bit 10 in that drill bit 10a has multiple guide projections 35a, 35b, 35c, and 35d concentric with axis 22a with equal radii extending and uniformly spaced apart around the periphery of side wall 18a at work end face 21a to facilitate cutting through layers of a composite work piece of different degrees of solidity or hardness. Drill bit 10a rotates in direction 20a about axis 22a to cut a hole in the work piece. FIG. 2B is a fragmentary sectional view showing drill bit 10a cutting into a layer of a multiple-layer work piece. The formation of guide projections 35a, 35b, 35c, and 35d on side wall 18a is analogous to the formation of guide projection 35 on side wall 18, except as indicated below. Guide projections 35a, 35b, 35c, and 35d reduce the propensity of side wall 18a to flex laterally as drill bit 10a passes through a softer material to a harder material and thereby maintain a uniform hole diameter through the multiple layers of a work piece. Although FIG. 2A shows guide projections 35a, 35b, 35c, and 35d at 90 degree angularly displaced locations, the number and angular separation of the guide projections can be selected depending on the properties of a work piece that would induce body flexure of the drill bit.
When formed of ordinary tool steel, drill bit 10 is particularly suited for the drilling of holes in wood, soft metals, such as aluminum and brass, and plastics, such as delrin(trademark) and nylon(trademark), and other like materials. The hole drilled can have a flat bottom (depending on the inclination of cutting edge 34 from axis 22 to side wall 18), will have an exceptionally smooth side wall, and will be accurately dimensioned. If cutting tooth 32 is formed of a harder material, such as a carbide, drill bit 10 can be used in harder metals, glass, porcelain, and other hard materials.
The cutting tool formed as a drill bit without a pointed tip as taught in U.S. Pat. No. 5,832,720 removes material from a work piece while providing exceptionally smooth side surfaces of holes drilled in the work piece. The drill bit lacks, however, a centering feature for accurately positioning the tool on the work piece. A user of the drill bit centers it by either visually estimating the location on the work piece surface where the periphery of the hole is to be cut or lightly contacting the drill bit against the work piece to slightly scratch its surface to show an outline of the periphery of the hole to be cut. Thus, the performance of the drill bit taught in U.S. Pat. No. 5,832,720 could be improved by the addition of a feature that allows convenient centering of the drill bit on the work piece.
An object of the present invention is, therefore, to provide a centering feature on a drill bit configured without a pointed tip for removing material from a work piece to provide drilled holes with exceptionally smooth side surfaces.
The present invention is a drill bit of a type shown in FIG. 1 or 1A that is implemented with a centering feature formed as an opening between first and second relatively angularly inclined portions of a work engaging end of the drill bit. The drill bit has a cylindrical body bounded by the work engaging end and a driver engaging end. The first portion of the work engaging end is defined by a cutting edge, and the second portion is defined by an end face side boundary running in a direction transverse to that of the cutting edge. The cutting edge and end face side boundary are offset from each other to create an opening that facilitates centering the axis of rotation of the drill bit on a work piece at a spot that represents the center of a hole to be drilled.
In a preferred embodiment, the drill bit has an elongate flute that extends along the length of the drill bit from the work engaging end to a location nearer to the driver engaging end. The flute includes generally flat trailing and leading faces that intersect at about a 90 degree angle to form a boundary line extending along the length of the drill bit near its axis of rotation. The cutting edge is positioned at the top of the trailing face of the flute at the work-engaging end and terminates at a medial termination point coincident with the axis of rotation of the drill bit. The leading face of the flute is recessed from the axis of rotation of the drill bit to form an opening between the end face side boundary of the leading face and the medial termination point of the cutting edge.
The opening permits the drill operator to view a punch mark or other indication on the work piece where the center of the intended hole is to be located. The drill operator positions the medial termination point of the cutting edge of the drill bit over the hole center location preparatory to contacting the drill bit against the work piece to cut the hole.